Trump quickly dismissed the move, saying the lawsuit is "totally without merit."

The case is based on the so-called "emoluments clause" of the Constitution, which says foreign powers and governments may not provide payments or gifts to the president without the approval of Congress.

Monetary damages not sought

According to The New York Times, which first reported the story Sunday, the suit does not seek monetary damages, but asks the court to stop the president from taking payments from foreign "entities."

CREW says because Trump has refused to completely divest from his businesses, "he is now getting cash and favors from foreign governments, through guests and events at his hotels, leases in his buildings and valuable real estate deals abroad."

The group's executive director, Noah Bookbinder, said in a statement, "We did not want to get to this point. It was our hope that President Trump would take the necessary steps to avoid violating the Constitution before he took office."

However Sherri Dillon, one of Trump's lawyers, insisted that the president was meeting all his ethical obligations at a press conference earlier this month. "No one would have thought when the Constitution was written that paying your hotel bill was an emolument," she said.

Trump announced at that news conference that he was handing over full control of his businesses to his two sons, Eric and Donald Trump Jr., along with one of his Trump Organization executives.

Eric Trump characterized CREW's lawsuit as "very, very sad," telling The New York Times it is "purely harassment for political gain."

Ethics experts have warned for some time that Trump could be in violation of the emoluments clause if he did not sell off all his assets and place the proceeds in a blind trust, in which he would not know how his money is invested. He has pledged not to discuss business with his sons and lawyers for the president point out that he is exempt from laws requiring other senior members of the executive branch to divest their holdings.

Most ethics analysts, however, say that anything short of a blind trust leaves Trump open to repeated questions about whether his actions as president are intended to benefit his financial interests.

When Trump sits down to negotiate trade deals with other countries, CREW says, "the American people will have no way of knowing whether he will also be thinking about the profits of Trump the businessman."

"President Trump has made his slogan 'America First," said Bookbinder. "So you would think he would want to strictly follow the Constitution's foreign emoluments clause, since it was written to ensure our government officials are thinking of Americans first, and not foreign governments."

Norm Eisen, an Obama administration ethics lawyer, is a member of CREW's legal team. He told The Times that the suit is intended in part to acquire access to Trump's federal tax returns so the group can evaluate the president's business dealings with foreign governments such as China and Russia.

Trump is the first U.S. president in decades who has refused to voluntarily make his tax returns public.